Bismaleimide polymers are a recently developed class of polymeric materials that have grown in importance as high strength materials that can perform at high temperatures. Polyimides prepared from bismaleimides are distinguished from other polyimides by their ease of processing, a function of their fully imidized form.
Bismaleimides are prepared by the condensation reaction of a diamine, e.g., methylenedianiline, with maleic anhydride and subsequent imide formation (cyclization). The condensation product tends to be crystalline and has a high melting point. While eutectic blends of different bismaleimides may be used to reduce the melting point of the polyimides formed from them, generally coreactants may be used to make bismaleimides processible, and to obtain the desired properties in the cured resins.
Bismaleimides are reactive because of the double bond on each end of the molecule. They can therefore react with themselves or with other compounds containing functional groups. Coreactants typically improve bismaleimide strength and toughness, and the ease of processing. When amines or other nucleophiles are used to co-cure a bismaleimide, the copolymer is less brittle than the homopolymer.
Bismaleimides are typically cured at temperatures of from 200.degree. C. to 350.degree. C. for 1 to 4 hours. Curing may be followed by a postcure step at about 350.degree. C. for 4 hours, to fully develop their properties. The glass transition temperatures of the polymers usually exceed 300.degree. C. The polymers may be used at temperatures exceeding 350.degree. C.
The monomers from which the bismaleimide polymers are obtained are those, for example, taught in U.S Pat. No. 3,297, 713 to Ladd. That patent discloses the preparation of symmetrical dithiobis (N-phenylmaleimides).
U.S Pat. No. 4,323,662 to Oba et al. discloses a broad range of bismaleimide-containing polymers and methods for forming those polymers.
While polyimide resins have been prepared by prior art methods and have demonstrated satisfactory properties for their intended uses, attempts to reuse the resins have not been successful. The cured polymers are generally insoluble, infusible, and not in any sense recoverable or modifiable. The art has heretofore failed to provide any method for reversibly crosslinking polyimide resins. Practically speaking, commercial polyimide resins cannot be recovered and the constituents thereof cannot be reused in new or modified resin structures.